In a general interior permanent magnet motor, as many magnet insertion holes as the number of poles are formed in advance in an axial direction through a rotor core formed by laminating and fixing a plurality of magnetic steel sheets punched into a predetermined shape. When the rotor is assembled, permanent magnets are inserted into corresponding magnet insertion holes. When the electric motor is operated, forces caused by the change in attracting force between the magnet and a tooth leading end and by the acceleration and deceleration act on the magnet, and hence the magnet may move laterally in a circumferential direction inside the magnet insertion hole. A large amount of movement of the magnet inside the magnet insertion hole causes vibration and noise, which further causes wearing, cracking, and chipping of the magnet. Therefore, in the vicinity of the lateral ends of the magnet insertion hole, steps or projections are often formed as lateral magnet stoppers against the movement of the magnet.
On the other hand, when the magnet stoppers are formed in the magnet insertion hole, the distance between the edges of the hole is reduced in this part, and thus the magnetic flux passes across the magnet more easily as compared to the case in the periphery thereof. Therefore, there has been a problem in that, when the magnet is exposed to a magnetic field caused by a winding current of a stator, the magnet tends to demagnetize in the vicinity of the magnet stoppers of the magnet insertion hole.
In a normal case, the lateral magnet stoppers against the movement of the magnet are not necessary in all of the laminated steel sheets, and it is sufficient that the lateral magnet stoppers be formed in only at least two steel sheets spaced apart in the axial direction. The necessary number thereof depends on the force of the magnet that tries to laterally move and the strength of the magnet stopper per sheet. In view of this, in order to reduce the demagnetization to be caused when the magnet stoppers are formed, the following method is conceivable. The magnet stoppers are formed in, among the plurality of magnetic steel sheets forming the rotor core, only at least two steel sheets spaced apart in the axial direction, and the magnet stoppers are not formed in the other steel sheets. In this manner, the use number (ratio) of steel sheets that may easily cause the demagnetization of the magnet is reduced.
As the configuration relating to such a method, for example, there is known a configuration disclosed in Patent Literature 1. In the configuration of Patent Literature 1, two types of steel sheets, that is, a steel sheet having a magnet insertion hole with a large lateral width in the circumferential direction (referred to as “steel sheet A”) and a steel sheet having a magnet insertion hole with a small lateral width in the circumferential direction (referred to as “steel sheet B”) are prepared, and the position of the magnet is regulated by two or more steel sheets B.